I tried a different cord and it started working again. Additionally i've had this work for me (see below)

Try This:

Unplug all USB devices

Turn off Ext Hard drive

Shut down computer

Power on ext hard drive (then wait like 1 min)

Turn on computer

Wait for windows to finish loading

Plug in the USB cord from the ext hard drive to your computer (may take up to a minute for computer to recognize it)

If it doesn't recognize it then keep trying it in different USB ports

If it still won't recognize get all the latest windows update and reboot. If that doesn't solve your issue try your external hard drive on some other computers, check to see if drive needs special drivers installed (which most don't).From SimpleTech's website: "1) I have the drive plugged in but I cannot find the drive in "My Computer", why?The most likely cause would be not enough power provided by a single USB port on your computer. Unplug the portable drive from the cable and unplug the cable from the computer. Now, plug the cable into 2 USB ports directly on your computer. (If you have a desktop, you should plug the USB cable directly into the USB ports on the back of the computer. If this is a laptop, you should plug directly into a USB ports on the computer. Do not plug into a USB hub or docking station). Then plug the drive into the cable. In a few seconds you should see the drive listed in the "My Computer" window as a SimpleDrivePS."

If all that fails you need to RMA (Return to Manufacturer) your external hard drive. Most hard drives have a pretty decent warranty on them. Your simpletech 250GB has a 3 year warranty on it. Info on the warranty can be found at http://www.simpletech.com/support/warranty/. If you want to RMA it, click the "Technical Support / RMA Request" on the left hand side of that page.

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if you have SATA HD, and can you see the drive in the BIOS? if yes, good and let's troubleshoot. if no, and you've definitely connected (and tried re-connecting) data and power cables, the HDD could be bad.

if you see the drive in the bios then try to install windows vista or windows 7, because in some of systems SATA drives can not be detected by XP

With the flash drive plugged into your computer, go to start>settings>control panel>administrative tools>computer management>disk management
Your flash drive should be there. Right click it and select "change drive letter and paths." Click on "change." Change the drive letter.
Just follow the directions to finish.

Sometimes, if you turn the computer on, insert the flash drive, and reboot, the drive will show up, it is detected on start along with your other drives.

(I recommend starting at the end of the alphabet (Z) and work backwards when selecting the drive letters for flash drives, that way, in the future it will be easier to recognize them.)

When connecting to the desktop be sure to plug it into the back. The back usb ports are high speed and should recognize the flash drive. If you go into the my computer is the flash drive shown there?. If it is make sure you format the drive to be compatible with xp. I am noticing a lot of complaints about this type of flash drive, there could be a driver issue. Try to visit the manufacturer website and get the most up-to-date driver.

Ok Go into your hardware device manager and uninstall your Universal Serial Bus Controllers and if there is mass storage device also uninstall it. Click on action -> scan for hardware changes, then it should automatically install the drivers back now it should detect.

From what I can tell from the Seagate/Maxtor site, the drive will work in Vista...but not the software it comes with. when you plug it in, does Vista recognize and install drivers for a "Removable Drive"? If so you should see it list in Windows Explorer, and just be able to manipulate it like a hard drive. It also notes that you should use Vista's built in backup utility instead of the Maxtor one. Here is a link to the Vista Compatibility that I got this information from. Hope this helps.